Our ability to exhibit self-control to avoid cheating or lying is significantly reduced over the course of a day, researchers have found.

"As ethics researchers, we had been running experiments examining various unethical behaviours, such as lying, stealing, and cheating," researchers Maryam Kouchaki of Harvard University and Isaac Smith of the University of Utah's David Eccles School of Business explained.

"We noticed that experiments conducted in the morning seemed to systematically result in lower instances of unethical behaviour," they said.

Knowing that self-control can be depleted from a lack of rest and from making repeated decisions, researchers examined whether normal activities during the day would be enough to deplete self-control and increase dishonest behaviour.

In two experiments, college-age participants were shown various patterns of dots on a computer. For each pattern, they were asked to identify whether more dots were displayed on the left or right side of the screen.

Participants were not given money for getting correct answers, but were instead given money based on which side of the screen they determined had more dots; they were paid 10 times the amount for selecting the right over the left.

Participants therefore had a financial incentive to select the right, even if there were unmistakably more dots on the left, which would be a case of clear cheating.

In line with the hypothesis, participants tested between 8 am and 12 pm were less likely to cheat than those tested between 12 pm and 6 pm - a phenomenon the researchers call the "morning morality effect."

They also tested participants' moral awareness in both the morning and afternoon.

After presenting them with word fragments such as "_ _RAL" and "E_ _ _ C_ _" the morning participants were more likely to form the words "moral" and "ethical," while the noon participants tended to form the words "coral" and "effects," lending further support to the morning morality effect.

The researchers found the same pattern of results after tests on a sample of online participants from across the US.

Participants were likely to send a dishonest message to a virtual partner or to report having solved an unsolvable number-matching problem in the noon, compared to the morning.

They also discovered that the extent to which people behave unethically without feeling guilt or distress, known as moral disengagement, made a difference in how strong the morning morality effect was. Those participants with a higher propensity to morally disengage were likely to cheat in both the morning and the afternoon.

The findings were published in Psychological Science, a journal of the Association for Psychological Science.